Objectives : High rate of dropout poses a practical issue for improving treatment outcome of bipolar disorders. The aim of this study is to investigate the rate and associated factors of dropout in patients with bipolar disorders. Methods : The follow-up duration and dropout rate of 323 patients with DSM-IV bipolar disorders registered at the Mood Disorders Clinic of Seoul National University Bundang Hospital between January 2005 and December 2007 was calculated. The factors associated with dropout were examined by telephone interview and chart review. Results : The dropout rate was 24.1%, 32.2%, 35.9%, 43.7%, 51.1%, and 58.9% at 1, 3, 6, 12, 24, and 36 months respectively. Age and gender did not significantly affect the pattern of dropout. ‘Denial of diagnosis of bipolar disorder’ was the most common cause of dropout in early phase of treatment (1-3 months). On the other hand, the dropouts in the later period of treatment (3-36 months) were mainly attributed to poor understanding of maintenance treatment. Conclusion : After the initiation of treatment for bipolar disorders, the dropout rate rapidly increased during the first 3 months and slowly increased around 12 months. Although overall rate of dropout in patients with bipolar disorders was comparable to other psychiatric disorders such as schizophrenia and major depressive disorder, the mean duration of follow-up appeared to be insufficient for recommended treatment. Differential pattern of factors leading to dropout according to treatment stage implicated the need of stage-specific psychoeducation program for patients with bipolar disorders.